Telephone system



y 9 4 s. B. WILLIAMS 2,248,146

' TELEPHONE SYSTEM v Filed July 2, 1938 3 Sheets-Sheet 1 INVENTOQ v 1 FIG. 2 H TF July 8, 1941. s w 1 s 7 2,248,146

Filed July 2 1938 3 Sheets-Sheet 2 imam a now! it Q

INVENTOR B 5.8.W/LL/AMS A TTORNE V y 1941- s. B. WDILLIAMS 2,248,146

TELEPHONE SYSTEM Filed July 2, 195a 3 Sheets-She et. 3

LAST SENDER FIG. 3

INTER. snvom MING/NIL INVENTOR 5.8. WILLIAMS ATTORNEY Patented July 8, 1941 TELEPHONE s Y's'rEiu Samuel B. Williams, Brooklyn; N; Ya assi'gnorto Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of-N'ew York Application July 2, 1938; Serial N6. 217,1"76' 7 Claims.

This invention relates to telephone systems and particularly to systems comprising automatic switches.

Objects of the inventionare a more efficient use of lines, trunks, or other circuits,. reduction in the number of circuits required in groups ter minating in the banks .of automatic switches, and the more efiicient operation of such switches.

This invention is a switching system comprising automatic switches and a plurality of circuits multipled to the terminals in the banks of these switches, each switch being arranged to hunt for and select a group of terminals havingat least one idle circuit therein and tohunt for an idle circuit in the selected group of terminals, the selection of any group of terminals by any one of the switches being efiective to guard the corresponding groups of terminals in the banks of the other switches until the one switch finds and seizes an idle setof terminals or until aset of terminals already tested by the one switch becomes idle after the one switch has advanced beyond this set of terminals.

The invention will be better understood by considering a switching system in which the invention is embodied, one such system being shown schematically in the drawings which form a part of this specification.- The invention is, however,-

not limited in its application to the particular embodiment herein disclosed but is; in general, applicable to any automatic switch arranged to hunt for an idle set of terminals in a plurality of groups of terminals.

Referring to the drawings:

Fig. 1 shows a subscribersline LI, 9. line-finder switch LF, a selector switch SI, a trunk T permanently associating the switch LB with the switch SI, a connector switch C and another subscriber's line L2; I

Fig. 2 shows a link LK comprising a trunkfinder switch TE and a sender selector switch SS; and a Fig. 3 shows portions of the first, an intermediate and the last register sender of a group accessible to the sender selector switch SS of Fig. 2.

The subscribers stations A and B may be of any known type comprising an impulse sender for use in controlling-the establishment of desired connections. The line-finder switch LF, selector switch SI and connector switch may be of the well known two-motion step-by-step type. For a description of the structure and the operation of such switches when used as selectors and connectors, reference may be had to pages 53 to 65, inclusive, of the secondedition of ffAi tomane Tel h y b mith nd Ca b F a complete disclosure of line and line-finder cir' cults and ,a description or the operation of a step by-steplineflinder switch, reference inay be had to Patent 1,799,651 granted to R. L. Stokly April 7,1931. Each of the switches LF, SI and C are represented by a set of brushes and as ingle set of terminals, The relays and other circuit elements associated with the switches LF and C have been omitted but the selector SI is shown indetail, Y The, switches TE and SS, whic h are also (if the well-known two-motion step-by-step type, are each represented by a set. of six brushes and a single, set of terminals. In addition to the brushes, and terminal banks, thetrunk-finder switch TF also comprises avertical commutator B8.and brush 61, a vertical stepping magnet 53. a rotary stepping magnet 55, a release magnet 59, a set of vertical cit-normal springs 58 and a. set of llthrotary step springs 60.. In addition to the brushes and terminal banks, the sender selector SS also comprises a vertical com mutator 98 and brush 9I,,.a vertical stepping magnet I3, a rotary stepping magnet 8I, arelease magnetl9, a set of vertical cit-normal springs :18, and a set of llth rotary step? springs 80. One hundred senders may be multipled to the one hundredsets of terminals in the banks of the sender selectors SS of all of the links LK. Assumingthat there are the same number of links as thereare senders, the links will be divided into groups, the trunk finders of each group having access to one hundred trunks T between line finders and selectors. Thus ten line finders willbe connected to the ten sets of terminals in eachlevelof the trunk-finder switch TF and ten senders will be connected to the ten sets of terminals ineach' level of the sender selector switch SS. e

fj Thej register senders may be similar to that disclosed in the patent to R. G. Richardson et al. No.'1,'691,4=10 granted November 13, 1928. The sender connected to the first set of terminals in the first level of switch SS is represented by an impulse receiving relay IOI, a release may I02, an off-normal or busy relay I03 and two associated.controlrelays I0 1 and I05, an impulse sending relay I II and a control relay H2. The intermediate sender is represented by its busy relay; I03 and associated control relays I04 and I05; and .the last sender is represented in like manner.

ductor 23 of trunk T, thence through the back contact of relay 22, conductor 29, and the inner back contact of relay 2|, to the winding of relay' l5 to cause the operation of relay i5. Relayl5 closes a connection from battery through the winding of relayl6, an upper front contact of relay i5, and conductor 46 to mark the test terminal of trunk T in the bank of the finder switch Tr and other trunk finders in the group. Relay i5 also closes a connection from grounded conductor 23, throughback contacts of relays 22 and 2 i, lowerfront contact of relay l5, and conductor 47 to operate group relay 48 which is common to a group of trunk circuits T. Relay l5 also closes a circuit for operating relay3l of selector SI; this circuit is traced from the ground on conductor back contact of relay 22, conductor 29, front contact of relay I5, back contacts of relays l6 and 35 and through the upper winding of relay '31 to battery. Relay 3| closes a circuit for operating slow-to-release relay 32. Relay 32 connects ground to conductor 29 and closes a circuit from battery through the upper winding of relay 31, back contacts of relays 30, I6 and H, retard coil 25, lower front contact of relay 32, back contacts of relays ll, iii and 36, lower winding of relay 3!, to ground at the normally closed contact or" the 11th rotary step springs 38. The'lower winding of relay 3! is short-circuited until relay 16 operates as hereinafter described. Relay 48 connects ground to a terminal of commutator 68 of trunk-finder switch TF to mark the level of the terminal bank in which are located the terminals to which the trunk T is connected. Relay 48 also connects ground through the start conductor 50, through precedin -busy finders, if any,'in the group, through normally closed contacts of relay 5? and the 11th rotary step springs 60, and through the winding'of relay 5| to battery. Relay 5| operates, connecting ground through its inner front contact to conductor 10, thence through a back contract of relay 8'! of sender selector switch SS to operate relay 1|; and relay H connects ground to conductor and is held operated until relay 81 operates as hereinafter described. Relay 5| also closes a circuit from ground at its inner front contact, through the back contacts of the vertical and rotary stepping magnets 53 and 55, lower winding of stepping relay 52, to battery at a back contact of relay 51. The stepping, relay 52 operates, closing a. circuit, which includesa back contact of relay 54, for operating the vertical stepping magnet 53. The operation of magnet 53 raises the brushes 6| and 66 and commutator brush Bl to the first level. normal springs 58 are actuated as soon as the shaft moves out of normal position. The operation-of magnet 53 opens the circuit through the 'lowerwinding of relay 52, and relay 52 releases unless the calling trunk is connected to terminals in the first level of the finder switch; in which case; relay 52 is held operated in a circuit traced from ground at the inner front contact of the The vertical offgroup relay 48, through conductor 49, the first terminal of commutator 68 and brush 6'', upper winding of relay 54, back contact of the rotary stepping magnet 55, lower winding of relays 52, to battery at a back contact of relay 5''. Relay 54 is operatively energized by its upper winding in this circuit but is slow in operating. Relay 52 prevents further vertical stepping. When relay 54 closes its lower front contact, a circuit is completed from battery at the middle upper back contact of relay 5! through the winding of magnet 53, lower winding and front contact of relay 54, to ground at relay 5|. The vertical stepping magnet is not operatively energized in this circuit but relay 54 is held operated independent of the commuator brush 61. If the trunk T is connected to terminals in a level other than the first, relay 52 releases when magnet 53 operates; the release of relay 52 causes the release of magnet 53; the release of magnet 53 causes the reoperation of relay 52; and the vertical stepping magnet 53 is thereupon operated to step the brushes of the trunk-finder switch to the second level. This cycle of operations is repeated to advance the brushes step-by-step until the calling level is found, at which time relay 52 is held operated and relay 54 is operated as above described, The complete operation of the relay 54 closes a circuit, which includes the contacts of the stepping relay 52, for operating the rotary steppingmagnet 55. The operation of magnet 55.rotates the brushes 6| to 66 into engagement with the first set of terminals in the level, the brush 61 being. moved out of engagement with the commutator terminal. The operation of magnet 55 also causes the release of the stepping relay 52. If the trunk T is connected to the first set of terminals in th level, relay 52 is held operated and relay 56 is operated when brush 66 encounters the test terminal of trunk T; but if the trunk for which the brushes are hunting is not connected to the first set of terminals in the level, relay 52 releases causing the release of magnet 55; whereupon relay 52 reoperates and magnet 55 is .reoperated to step the brushes to the next set of terminals. This cycle of operations is repeated until brush 65 encounters the test terminal of trunk T, at which time a circuit isyclosed from battery through the winding of relay l6 of trunk T, a front contact of relay I5,

conductor 46, terminal and brush 66 of trunk finder TF, uppermost back contact of relay 51, lower winding of relay 56, upper winding of relay 52, to ground'at relay 5|. Relays l6 and 56 are operated and relay 52 is held operated in this circuit. Relay 52 holds magnet 55 to prevent further advance of the brushes. Relay 56 closes a circuit from battery through the lower winding of relay 51, lower front contact of relay 56, to ground at the upper contact of VON springs 58. The operation of relay 5'! causes the release of relays 5|, 52 and 54, and the release of magnet 55. Relay 5'! also extends the starting conductor 50 to conductor 69 leading to the next finder in the group. If the sender selector SS has not yet found an idle register sender, as hereinafter described, relay 56 is held in a circuit through its locking winding and upper front contact, lower back contact of relay 96, and through conductor 10, to ground at a front contact of relay H. If-an idle register sender has been seized and relay 9|] operated as hereinafter described; relay 56 -is held, through the lower front contact-of relay and brush 96, to ground in :the sender.

The aforementioned operation of relay I6, when the trunk finder TF connects with the trunk T, closes a circuit through front contacts of relays I6 and I5 and conductor 29 for operating relay 2|. Belay 2| disconnects ground from conductor 41 to release; group relay 48. Relay I6 also connects the windings of line relay 3| of selector SI over conductors 43 and 44 to the brushes I53 and 64 of trunk finder TF. If at this time the sender selector SS has not yet completed selection of an idle register sender, relay 90 is normal and relay 3| is held operated by its upper winding through conductor 44 and brush 64, to ground at the lower front contact. Relay I locks to ground at the of relay. H. front contact of relay 3| and closes a locking circuit for relay I to the same ground; and this ground is further extended over conductor 46, through brush 'fifi' and the front contact of relay 51 and through the back contact of relay 90 to conductor 10, if an idle sender has not yet been seized, or through the front contact of relay 90 to conductor 89, if an idle sender has'been seized as hereinafter described. f

When relay 5| operates as hereinbefore described to start the operation of the trunk finder TF, it also connects ground through its inner front contact to conductor E0 to cause the operation of relays II, 12 and H of the sender selector S3. The connection between conductor and the winding ofrelay II includes a back contact of relay 8?; the connection between conductor l0 and the winding of relay l2 includes a back contact of relay 8?, conductor 84, andnormally closed contacts of the vertical off-normal springs I0; and the connection between conductor 10 and the winding of relay 1! includes conductor 84 and the lower back contact of relay 15. lhe operation of relay I connects ground to conductor 10 so as to hold relays 56 and II operated until relayS'I operates as hereinafter described. If the trunk finder TF finds trunk T before an idle sender is'selected, the ground thus connected to conductor I0 also extends through the back contact of relay 00, front contact of relay 51, and brush 66 to conductor 46 to hold relays I5 and I5 operated; and ground at'the outer lower front contact of relay is connected'through brush S4, conductor 4 a front contact of relay I6, and a back contact of relay 30 to the upper winding of relay 3| so as to hold relays 3| and 32 operated. Relay 12 looks under control of the vertical stepping magnet 13 and closes a circuit from battery through the winding of vertical stepping magnet 13, winding of slow-torelea-se relay I4, lower front contact of relay 12, conductor 84, back contact of relay 87-, conductor 0 to ground at relay 1 I. Relay I4 and the magnet 13 operate. Magnet I3 steps the brushes 9| to 06 and commutator brush 8? up to the first level and also causes the release of relay .12. Relay M is slow in releasing and remains operated until vertical stepping is completed; The vertical oif-normal springs 18 are operated when the'shaft of switch SS moves out of normal position, closing a circuit for operating relay 82; and relay 82 locks through back contacts of rotary stepping magnet SI and relay 01. If upon engagement of commutator brush 91 with any level terminal, the engaged terminal is guarded against selection by a ground potential connected thereto, relay i2 is reoperat'ed, when magnet 13 releases, by the current in a circuit which includes the inner upper back contact of relay I5, brush 9'! and the guarded terminal. The re magnet 13 and the brushes are thereby advanced to the next level. Relay I2 and magnet 13 continue to reoperate and advance the brushes stepby-step until brush 91 engages an unguarded level terminal at which time a circuit is closed for operating relay I5 from battery through relay I00 or corresponding relay of another level, brush,9'|, back contact and upper winding of relay 15, lower front contact of the VON springs 18, conductor 84 and back contact of relay 8! to ground on conductor I0. Relay I00 ismarginaland does not operate at this time. I

Before describing the further operation of the sender selector SS, the variousconditions which exist when a levelis guarded against selection willbe described.- First-,all of the senders in the level maybe'busy, in which case the level relay I00 is being held operated in a chain circuit through a front contact of the busy relay {I03 in each of the senders in that level. Second, there may already be a sender selector-hunting in the level; in which case, ground is connected through the back contacts of relays I04 and I05 in the first idle sender in the level and through the front co'ntacts of the corresponding relays in precedingbusy senders in the level to hold relay I00 and to guard the commutator terminal against seizure. If, however, a sender, which'a hunting sender selector has already tested and found busy,'b'ecomes idle before this selector has found an idle sender in the level, the release of its relay I93 causes the release of relays I04 and I05 and disconnects the guarding ground potential from the locking contact-of relay I00 andfro'm'the level terminal. Since relay I04 is slow in releasing. relay I00 releases before the armature of relay I04 engages its back contact. Thedisconnection of the guarding ground potential from the level terminal allows another sender selector to start hunting in thislevel,- although the first selector hunting therein has not yet found and seized an idle sender. I Y When an unguarded level is reacheda'nd magnet 13 has released, relay I2 is connected in series with the upper winding of relay I5 to grounded conductor 10. The current in this circuit holds relay 15 but does not reoperate marginal relay 12 Relay "I5opens the circuit through thewind-- ing ofrelay TI and connects the winding of relay I6 in series with brush 9'! and relay I00. Relay "is slow in releasing and, if the commutator brushes 91 of two sender selectors should be testing any level at the same time, relays l6 ofthese sender selectors will fail to operate on account of the increased drop'in potential in thewinding enemy I08} and, in this case when relay TI releases"; relay "I2 is reoperated through the uppermost back contact of relay H, the back contact of relay- It, the lower front contact of springs 18 andconductor 84 to cause the reoperation of magnet 13- and continued energization of relay '|4.- When brush 91 engages the terminal-of a level in whichthere is an idle sender,-' in which noot er sender selector is hunting-and at a time when no other sender selector is testing this l'eveL-rel'ay l0 operates'prior to the release of relay 'I'I, preventing the reoperation of relay I2; and in this case,- the releaseof relay IT closes a holdingcircuit for relay 15 from battery through the upper winding of relay 15, front contact'of relay 16, lower front contact or relay 15, and conductor-M-toground at relay 1!. Therelease of 'relavllc'auses the release'of relay 510 and temperance or level relay H31! i the circuit for operation of relay "causes the reoperation of operating relay i is traced through brush 91, the inner upper front contact of relay I5, inner back contact of relay I1, lower front contact of the VON springs 18, conductor 84, and through a back contact of relay 8! and conductor 10 to ground. Relay I00 locks through front contacts of relays I03 and I05 of busy senders and the back contacts of relays I33 and I64 of the first idle sender in the level. Relay 75 is held by its upper winding in series with the winding of marginal relay 12 until seizure of an idle sender is completed.

When vertical stepping is completed and relay 14 releases, the rotary stepping magnet 8| is operated by the current in a circuit which includes the lower front contact of relay 82, inner back contact of relay 14, lower front contact of VON springs I8 and conductors 84 and I0. The operation of magnet 8| rotates the brushes 9| to 96 into engagement with the first set of terminals in the selected level and causes the release of relay 82. The release of relay 82 causes the release of magnet 8|. The commutator brush is rotated out of engagement with its bank when magnet 8| operates so that the holding of level relay I00 is controlled solely by the busy relays of the senders in the level. If the sender connected to the first set of terminals is idle, relay 82 does not reoperate and relay 81 operates; but if this sender is busy, there is a guarding ground potential connected to the terminal engaged by brush 96 which short-circuits the winding of relay 8! and effects the reoperation of relay 82. If relay 82 reoperates, it causes the reoperation of magnet 8| and the advance of the brushes to the next set of terminals. When an idle sender is reached, relay 8! is operated by the current in a circuit from battery through the winding of relay 82, back contact of magnet 8|, winding of relay 81, upper contact of VON springs 78, to ground at a front contact of relay II. Being marginal, relay 82 does not r-eoperate in series with relay 81. Relay 81 connects the operating ground through its inner upper front contact to conductor 89 and brush 96 to temporarily guard the sender against seizure by another sender selector; and connects this ground to the winding of relay 90. Relay 8! also opens the circuit through the winding of the slow-to-release relay 1 I. The operation of relay 90 connects brushes 6| to 66 of finder TF to brushes 9| to 96 of sender selector SS. If at this time the trunk finder TF has found the trunk T and the line finder LF has found the calling line, the impulse relay IOI of the selected sender is operated in a circuit which includes brushes 9| and 92 of sender selector SS, front contacts of relay 90, brushes 6| and 62 of trunk finder TF, conductors M and 42, front contacts of relay I5 of trunk T, brushes II and I2 of line finder LF and the calling subscribers line loop. Relay IOI causes the operation of relay I02 and relay I02 connects a busy and holding ground potential to the terminal engaged by brush 96 to hold relays 56, 81 and 90 of the link LK and relays I5 and I6 of trunk T. Since relay II is slow in releasing, relay I 02 connects ground to' conductor 89 before relay II releases. If, however, the trunk finder has not yet found the calling trunk when an idle sender is seized and relay 90 operates, relays I0| and I02 do not operate and relays 81 and 90 are held, after relay releases, over conductor 89 through the lower front contact of relay 90 and a back contact of relay 51 to ground at the front contact of relay 5|. As soon as the trunk finder TF finds the trunk T and relay 51 operates, con

ductor 89 is connected with conductor 46 so that ground from the front contact of relay 3| holds relays 56, 8! and operated until relays MI and I02 are operated over the calling subscribers loop.

With relay I6 operated, due to the connection of the brushes of trunk finder TF to trunk T, and relay 90 operated, due to the connection of the brushes of sender selector SS to an idle sender, the windings of re1ay 3| are connected through front contacts of relay I6, conductors 43 and 44, brushes 63 and 64, front contacts of relay 90, brushes 93 and 94, to the closed contacts of the impulse sending relay III in the selected sender RS.

When the calling line has been found by the line finder LF and extended through the contacts of relay I5, the trunk finder TF and the sender selector SS to relay |0I of the sender, dial tone is transmitted to the calling subscriber in usual manner. When the called number is dialed, relay IOI responds to each impulse and the called number is registered. Relay I02 is slow-to-release and remains operated during response of relay I0| to dial impulses. The impulse sending relay III is thereupon operated to control the selective operations of selector SI, intermediate seelctors, if any, and connector 0. Since the operation of two-motion step-by-step selectors is well known the operation of selector SI will be only briefly described. Relay 3| responds to the first train of impulses transmitted by relay of the sender, each closure of the back contact of relay 3| being effective to operate the vertical stepping magnet 33 and thereby raise the brushes up to the desired level. Relay 34 operates in series with magnet 33 and being slow in releasing remains operated until vertical stepping has been completed. Relay 32 is slow-to-release and remains operated while impulses are being received. The operation of the vertical off-normal springs 39 closes a circuit for operating relay 36 and relay 36 locks under control of rotary stepping magnet 35. When relay 34 releases, magnet 35 operates, releasing relay 36 and. advancing the brushes into engagement with the first set of terminals in the selected level. After magnet 35 releases, relay 36 reoperates if the test terminal of the first set is connected to ground; and in this case magnet 35 reoperates, stepping the brushes to thenext set of terminals. When an unguarded set of terminals is encountered, relay 30 operates in series with relay 36 but relay 36 is marginal and does not reoperate. Relay 30 connects ground from the front contact of relay 32 to the test brush to mark the selected trunk as busy and extends the impulse circuit through the brushes of selector SI to the line relay (not shown) of the seized selector or connector. The operation of relay 30 causes the release of relay 3| but relays 32, I5 and I6 hold over conductors 28, 46, and 89 to ground at the front contact of relay I02.

The sender thereupon transmits the trains of impulses required for selectively operating any intermediate selectors and the connector C to establish connection with the called line L2. When all impulses have been transmitted, relay I I2 is operated, disconnecting the holding ground from conductors 89 and 46 causing the release of relays I5, I6, 56, 81 and 90. The release of relay I5 disconnects the calling line from conductors 4| and 42 and connects the line to the windings of line relay 3| of selector SI. Relay 3| reoperates, holding relay 32 operated and re- 'laytE maintains the connection of the busy roundto-the testbrush of the-selector SI until the connection is 'released by the calling subscriber. A supervisory bridge is closed by the release of relay I6 through the lower front contact of relay 32 and the retard coil 20 for holding the line relay (not shown) of the connector C.

When relay I releases, relays HH and I02 of the register sender releaseand the register sender is thereupon restored to normal. When ground is disconnected from conductor 89, due to release of relay 'I02 of the sender, relays 56, 81 and 90 release. The release'ofrelayB'I closes a circuit for operating the release magnet "IQ of the sender selector SS and the release of relay'56 closes a circuit for operating the release magnet 59 of the trunk finder TF. When these switches reach normal, the return of the .VON springs 58 and I3 to normal opens the circuits of the release magnets. Relay 5'! of the trunk finder TF releases when this switch reaches normal unless the group relay 48 is being held by another call.

If all senders are busy, the brushes of the sender selector are raised to the tenth level and rotated therein to the eleventh rotary position in which the 11th rotary step springs are operated. Relays ?5 and 8'! not having been operated, the release magnet 19 is energized and the sender selector is restored to normal. On reaching normal, the hunting operation of the switch is resumed.-

When the calling subscriber replaces the receiver, relays 3i and 32 release, thereby causing the release of cut-through relay 30 of selector SI, the release of the cut-through relays of intermediate selectors, if any, the release of the cutthrough relay 3!! of trunk T and the release of the cut-through relay, not shown, of the line finder LF. The release of relay 3!] closes the circuit for operating release magnet 31, restoring the brushes of selector SI to normal. The connector and line-finder switches are restored to normal in usual and well-known manner.

If the call is one which the sender routes to an operator, the relay ID! in the sender is operated in a circuit local to the sender as soon as the switch SI selects a set of terminals which are connected to a trunk to an operator. The circuit for operating relay I! is traced from ground through resistor IE5, front contact of relay I01, terminal and brush 95 of switch SS, front contact of relay 9E1, brush 65 and terminal of switch TF, conductor 45, through the lower windings of relays 22 and I! in parallel to battery. Relay 22 is marginal and does not operate. Relay II looks through its upper winding and inner upper front contact, back contact of relay 22, conductor 29, lower front contact of relay 3!], test brush of selector Si to a holding ground in the trunk to the operator, which trunk is not shown in the drawings. Relay I! disconnects relay 3| and connects the calling line to and through front contacts of relay 3B and the upper brushes of switch Si, to the talking conductors of the trunk. Relays 3I and 32 release but relay 30 is held through the test brush to the holding ground in the trunk to the operator. When the connection is released, the disconnection of the holding ground causes the release of relays I1 and 30 and the return of switches LF and SI to normal.

While the invention is herein applied to a sender selector switch, it is, of course, applicable to any circuit hunting switch which automatically operates to select an idle set of terminals in any one of a plurality of groups.

passed becomes idle.

What is claimed is: g

1. A switching system comprising a plurality of lines divided into groups, selectors, each selector arranged to select'a group having onefor more idle lines therein and to hunt 'for an idle line in a selected group, and means including a group terminal for marking a group busy upon selection of the group by any one of said selectors and for maintaining said marking until an idle line has been selected by the selector in question.

2. In a switchingsystem according to claim 1,

means forterminating the'marking of a group as busywliile a selector is hunting for an idle line thereinwhen and if a line already'tested and 3. In combination, selectors, each selector comprising a set of brushes and a bank of terminals,

the terminals of said bank being divided into groups, a primary magnet for advancing the brushes to select a group of terminals, a group testing brush, a group test terminal for each of said groups of terminals, a secondary magnet for advancing the brushes to select an idle set of terminals in a selected group, and means for starting the operation of the selector, groups of lines, each group being connected to the terminals of a different one of said groups of terminals in the bank of each of said selectors, group relays, one for each of said groups of lines, each group relay being connected to the associated one of the group test terminals of each of said selectors, busy relays for each of said lines, means comprising busy relays of all lines in a group for connecting a guarding potential to the associated group test terminals to prevent selection of the group, means comprising the group relay of a group of lines for connecting the guarding potential to the associated group test terminals to prevent selection of the group, and a circuit closed upon selection of a group of terminals by one of said selectors to operate the group relay to guard said group against selection by another of said selectors.

4. In a combination according to claim 3, means for disconnecting the guarding potential from the group test terminals of a group while one of said selectors is hunting therein if and when one of the lines already tested and passed by becomes idle, said disconnecting means comprising a relay individual to said one of the lines.

5. In combination, selectors, each selector comprising a set of brushes and a bank of terminals, the terminals of said bank being divided into groups, a primary magnet for advancing the brushes to select a group of terminals, a group testing brush, a group test terminal for each of said groups of terminals, a secondary magnet for advancing the brushes to select an idle set of terminals in a selected group, and means for starting the operation of the selector, groups of lines, each group being connected to the terminals of a different one of said groups of terminals in the bank of each of said selectors, group relays, one for each of said groups of lines, each group relay being connected to the associated one of the group test terminals of each of said selectors, a circuit closed upon selection of a group of terminals by one of said selectors for operating the associated group relay, and a circuit closed by said group relay for connecting a guarding potential t0 the associated group test terminals of the other selectors to prevent selection of the group by another selector.

6. In a combination according to claim 5, means for disconnecting the guarding potential from the group test terminals of a group while one of said selectors is hunting therein if and when one of the lines already tested and, passed by becomes idle, said disconnecting means comprising a relay individual to said one of the lines.

'7. In combination, selectors, each selector comprising a set of brushes and a bank of terminals, the terminals of said bank being divided into groups, a primary magnet for advancing the brushes to select a group of terminals, a group testing brush, a group test terminal for each of said groups of terminals, a secondary magnet for advancing the brushes to select an idle set of terminals in a selected group, and means for starting the operation of the selector, groups of lines, each group being connected to the terminals of a difierent one of said groups of terminals in the bank of each of said selectors, group relays, one for each of said groups of lines, each group relay being connected to the associated one of the group test terminals of each of said selectors, busy relays for each of said lines, a circuit closed upon selection of a group of terminals by one of said selectors for operating the associated group relay, and a circuit closed by said group relay for connecting a guarding potential to the associated group test terminals of the other selectors to prevent selection of the group by another selector, said last-mentioned circuit comprising a back contact of a busy relay of the first idle line in the group and a front contact of a busy relayof each of any preceding busy lines in the group.

SAMUEL B. WILLIAMS. 

